I have no idea how the US tax system works. But how about this thought: from saying you are taxable if running servers in some state, it wouldn't be a big leap saying "your javascript code runs in browsers on computers that are in our state and you are therefore taxable"...

A pretty big one.

A server is, arguably, a physical location serving products. Javascript running in a browser is the product served. Going with a tried-and-true car analogy, a state can't tax a car manufacturer because the cars drive on their roads, but they can tax them if they produce or sell cars in that state.

It is not always that easy to distinguish servers and clients.

What about P2P systems such as Skype?

I can't imagine Rackspace would be happy with the impact that would have on their clients...